Smallest field I've flown .60 sized planes on is 90' x 300', with a tree-line on each side of the field, aligned with the flightline. crops surround all four sides, and if the farmer plants corn, it makes things even more interesting!

My sport/3D planes do fine, but I have a Mustang with retracts that I don't really want to fly there, I like to have a little more runway to bring her in.

In addition to field size, you must consider your safe & allowed flyover area. You'll want at least 35 acres or more for .60 sized planes. My club (with the 90'x300' runway) has about 37 acres of fly-over area.

http://www.flyingcropdusters.org